An automatic cigarette packing machine comprises a number of conveyors, each for feeding cigarettes successively along a respective path, and having a number of respective movable members. Since it is often necessary to determine the exact instant the movable members move into a given position, e.g. so as to correctly time operation of an operating device interacting with the cigarettes carried by the movable members, the automatic machine comprises one or more reading devices, each for determining the position of the movable members of a respective conveyor.
Reading devices of the above type are typically optical, and comprise a beam emitter, and a receiver connected to the emitter and for determining the instant the beam is interrupted by passage of a movable member. Optical reading devices, however, are relatively inaccurate, and require frequent cleaning to prevent tobacco powder from settling on the emitter or receiver and so impairing operation of the device.